The present invention relates to thin film microstrip circuits and more particularly to a thin film circuit that is compatible with lead-tin solder and which provides low insertion loss at X-band frequency.
Thin film microstrip circuits are presently fabricated primarily from metallization systems consisting of gold as the main conductor. A typical system has a layer of tantalum acting as an adhesive layer and resistor metal followed by a thin layer of chromium acting as a diffusion barrier, and finally a layer of gold acting as the main conductor. Because of the high conductivity of gold, the insertion loss at X-band frequency is low. The inherent chemical inertness of gold also makes it desirable as the outer metallization when considering oxidation and corrosion problems. The photofabrication processes for etching gold based thin film circuits are well defined and capable of yielding circuitry with line widths and spacings of 1 mil for gold films on the order of 50,000 Angstroms thick. The etchants used respectively for etching gold, chromium and tantalum do not interact with the other metals, and therefore high resolution of etched circuitry is possible. However, gold based circuitry must be soldered with lead-indium solder because of solder leaching of the gold by tin based solders. Recent discovery of corrosion and cracking of lead-indium solder joints has made it necessary to develop alternative metallization systems which are compatible with lead-tin solders, but which also exhibit acceptable transmission reflection parameters at high frequencies and which are still compatible with fabrication of tantalum thin film resistors.
Proposed solutions, such as tantalum-nickel-gold or tantalum-palladium-gold are desirable interms of lead-tin solderability and their resistance to solder leaching. However, because of the use of lower conductivity metals such as nickel and palladium in closer proximity to the substrate surface than the gold, insertion losses at X-band frequency are relatively high.